Ultrasonic heating such as high intensity focused ultrasound (HIFU) is utilized for certain therapeutic applications. As disclosed in commonly assigned International Application PCT/US98/1062, published as International Publication WO/98/52465 the disclosure of which is hereby incorporated by reference herein, HIFU heating typically is conducted using an ultrasonic emitter having an array of transducers. The transducers are actuated with a drive signal so as to emit ultrasonic waves. The relative phasing of the waves is controlled by the physical configuration of the array and the phasing of the drive signal. These factors are selected so that the ultrasonic waves tend to reinforce one another constructively at a focal location. Tissue at the focal location is heated to a greater extent than tissue at other locations. As described, for example in commonly assigned U.S. Pat. No. 6,461,314 and in commonly assigned U.S. Pat. No. 6,492,614, the disclosures of which are also incorporated by reference herein, HIFU may be applied by transducer arrays such as arrays of polymeric piezoelectric transducers. These arrays can be mounted on a probe such as a catheter which can be introduced into the body, for example, as in a cavernous internal organ or within the vascular system to perform cardiac ablation.
Contraction or “beating” of the heart is controlled by electrical impulses generated at nodes within the heart and transmitted along conductive pathways extending within the wall of the heart. Certain diseases of the heart known as cardiac arrhythmias involve abnormal generation or conduction of the electrical impulses. One such arrhythmia is atrial fibrillation or “AF.” Certain cardiac arrhythmias can be treated by deliberately damaging the tissue of the cardiac wall along a path crossing a route of abnormal conduction. This results in formation of a scar extending along the path where tissue damage occurred. The scar blocks conduction of the electrical impulses. Such a scar can be created by conventional surgery, but this entails all of the risks and expense associated with cardiac surgery. Alternatively, the scar may be made by application of energy such as heat, radio frequency energy or ultra sonic energy to the tissue that is to be scarred. Scarring the tissue by application of energy is referred to as cardiac ablation.
Commonly assigned U.S. Pat. No. 6,635,054, the disclosure of which is incorporated by reference herein in its entirety discloses thermal treatment methods and apparatus. The disclosed apparatus includes collapsible ultrasonic reflector. The reflector incorporates a gas-filled reflector balloon, a liquid-filled structural balloon and an ultrasonic transducer disposed within the structural balloon. Acoustic energy emitted by the transducer is reflected by a highly reflective interface between the balloons and focused into an annular focal region to ablate the cardiac tissue.
Commonly assigned U.S. Patent Application Publication No. US 2004/0176757, the disclosure of which is incorporated by reference herein in its entirety, discloses cardiac ablation devices. The disclosed devices are steerable and can be moved between a normal disposition, in which the ablation region lies parallel to the wall of the heart for ablating a loop like lesion, and a canted disposition, in which the ring-like focal region is tilted relative to the wall of the heart to ablate curved-linear lesions.
Conventional methods and apparatus, including the methods and apparatus mentioned above, utilize a continuous mode power profile to ablate cardiac tissue in the treatment of atrial fibrillation. However, with the conventional methods and apparatus, the collateral tissue immediately adjacent to the intended ablation target can heat up to a temperature that may result in unwanted necrosis of untargeted collateral tissue.
This unwanted necrosis of collateral tissue results from excess temperature elevation, in the targeted tissue. Conventional systems deliver power in the continuous wave (CW) mode for the entire duration of the ablation cycle which sometimes results in temperature rises in the targeted tissue in excess of that needed to create necrosis. Heat from the target tissue is conducted to nearby collateral tissue and anatomical structures such as the phrenic nerve and esophagus. If the amount of heat energy is sufficiently high, than heat conducted from the targeted tissue to the collateral tissue results in elevated collateral tissue temperature sufficient to create unwanted necrosis.
Thus, there remains an unmet need for an optimized power delivery profile that quickly elevates the targeted tissue to temperatures resulting in necrosis, then maintains that temperature at a constant or near constant level for a period of time needed to achieve complete targeted tissue necrosis while, at the same time, ensures that heat conducted to adjacent collateral structures remain insufficient to cause unwanted or untargeted necrosis.